Field effect transistors (FETs) have widely been used as important switching devices or amplifying devices as well as bipolar transistors. Hitherto, transistors using silicon, and others have been put into practice, and applied to wide fields. A field effect transistor exhibits characteristics thereof by controlling the transportation of carriers in its semiconductor layer present between its source electrode and its drain electrode by the use of its gate electrode with an insulating layer interposed therebetween. The field effect transistor is called an n type in a case where the carriers are electrons, and called a p type in a case where the carriers are holes (a monopolar device). In contrast to the monopolar device, the field effect transistor capable of transporting both of electrons and holes is called a bipolar device.
In particular, devices wherein a metal oxide is used in an insulating layer, which are called MOS (metal oxide semiconductor) structures, are widely applied to logical gate devices, inverter circuits, memory devices, and others. Of the devices, well known are MOS-FETs having a thermally oxidized film of silicon dioxide on silicon.
About inorganic semiconductor devices, a typical example of which is Si, highly complicated producing steps are repeated many times; thus, enormous costs are required for the production thereof. Moreover, the production includes a step of high-temperature treatment; therefore, it is difficult to use a flexible plastic substrate or an organic semiconductor. In comparison thereto, in the case of organic transistors, an device using a plastic substrate can be produced. Thus, the organic transistors are expected as flexible and light transistors.
In recent years, attention has been paid to devices wherein organic material is used in an active layer, such as organic ELs, organic lasers, organic solar cells, and organic transistors. Advantages based on the use of organic material include advantages that various materials can be designed and many added values can be given. In the case of giving an organic transistor as an example, the following advantages are mentioned: high temperature treatment, which is essential for conventional Si processes, is not required; therefore, organic transistor can be fabricated on a plastic substrate, and added values that the transistor is flexible and light and is not easily broken can be given. Moreover, the production process can be made very simple and easy, and a semiconductor material soluble in a solvent can be yielded in accordance with material design. This makes it possible to apply thereto a printing process such as screen printing or inkjet printing. Thus, from the viewpoint of productivity and costs, organic semiconductors are very advantageous as compared with inorganic semiconductors.
The operation characteristics of field effect transistors are largely relative to the electrostatic capacity of their insulating layer, their device structures (channel length and channel width), and the carrier mobility of their semiconductor layer. In organic semiconductor materials, the development of materials having a high mobility has been actively made. Additionally, the development of organic semiconductor materials high in stability has also become important since there has been caused a problem of deterioration in device characteristics based on a change with the passage of time.
Patent Document 1: Japanese Patent Application Laid-Open No. 2001-94107
Patent Document 2: Japanese Patent Application Laid-Open No. 2002-198539
Non-Patent Document 1: Applied Physics Letters, 2001, vol. 78, p. 228
Non-Patent Document 2: Advanced Materials, 1999, vol. 11, p. 480